It is known to calcine limestone by passing it downwardly by gravity through heating stages of increasing temperature in direct contact with combustion products of a fuel such as natural gas, as shown for example in U.S. Pat. No. 3,796,791, issued Mar. 12, 1974 to Nielsen et al.
More recently, there has been disclosed a calcining process wherein the calcination heat is provided by combustion of hydrogen in direct contact with moist calcium hydroxide pellets and limestone fragments on a travelling grate; see U.S. Pat. No. 3,998,928, issued Dec. 21, 1976 to Stendel et al.